The strap lug is held in place by a screw under the top cover. To remove the cover, use a pair of pliers to unscrew the chrome ring which surrounds the shutter button, then lift off the winding lever and the washer below it. Then remove the three philips screws which hold on the top cover. After you remove the screws, the cover will lift off. You should be able to find the screw which holds on the strap lug somewhere under the top cover.
While the cover is off, you can clean the viewfinder and glass windows.
